Definition of "backbox"

The game is made exclusively in the new-type Williams cabinet design (a longer, lower styling with pedestal-mounted backbox).

1960 August 15, Billboard, page 69

Backbox animation was a steady attraction during the 1950s.

1997, Gary Flower, Bill Kurtz, Pinball: the lure of the silver ball

As if all of that were not enough to make this game special, it also comes with a board game in the backbox. Accomplishing success in the board game is about as important as playing good pinball toward achieving good scores (and it is the easiest way to get tokens to spit out at you).

2005, Kevin Strasser, The PinBotz Guide to the Greatest Pinball Machines of the 80s and 90s, page 50